1. Field of the Invention
The present invention relates to a wireless local area network (LAN), and more particularly, to a method of informing nodes included in the wireless LAN of how to avoid access collisions in a wireless LAN.
2. Description of the Related Art
Medium access control (MAC) using a carrier sensing multiple access with collision avoidance (CSMA/CA) method is used in a wireless LAN.
In the CSMA/CA method, physical carrier sensing and virtual carrier sensing are used for carrier sensing. In physical carrier sensing, a physical layer (PHY) determines whether a received power equal to or greater than a specific value is detected, and informs a medium access control (MAC) layer of whether a medium is in a busy or idle state, thereby sensing a carrier. In virtual carrier sensing, if a MAC protocol data unit (MPDU) can be correctly extracted from a received PHY protocol data unit (PPDU), a header field of the MPDU, that is, a duration/ID field, is analyzed, and the medium is deemed to be in the busy state during a scheduled time for using the medium. Stations determine whether the medium is in the busy state by using the two carrier sensing methods, and do not access the medium if in the busy state.
Referring to FIG. 1, a MAC header of a frame transmitted through a general IEEE 802.11 compliant wireless LAN includes duration information indicating a time between when the frame is transmitted and when an ACK frame is received to confirm that the frame is received. After receiving the frame, the stations analyze the MAC header so that medium access is not tried for a duration of time, thereby avoiding collision. According to a feature of a wireless medium, all stations connected through the wireless LAN can physically receive all frames transmitted in a radio wave coverage area even if the frame is sent to a particular station.
In FIG. 1, a frame exchange sequence is shown in which a single MPDU and an ACK frame are involved. However, a plurality of MPDUs may be included in the single PPDU as shown in FIG. 2A. Referring to FIG. 2A, a transmitting station and a receiving station exchange a request-to-send (RTS) frame with a clear-to-send (CTS) frame so that other stations do not access the medium for a predetermined time. The time obtained by the transmitting station when using an RTS/CTS exchange sequence is referred to as a transmission opportunity (TxOP). A network allocation vector (NAV) is a remaining time until the medium is available. Stations, except for the transmitting station and the receiving station, determine their NAV times using duration fields of a RTS frame, a CTS frame, and a data frame. Referring to FIG. 2A, the transmitting station sends a plurality of MPDUs included in a single PPDU. Furthermore, the transmitting station sends a plurality of PPDUs within a TxOP obtained by one RTS/CTS exchange sequence. In this case, data having a size larger than that of the general case of FIG. 1 is transmitted, and thus if an error occurs, more time is required to retransmit the data, thereby causing a longer NAV. If the longer NAV is determined, even when the transmitting station has no data to be sent, other stations do not access the medium for the remaining TxOP, which may cause a waste of channels. To avoid this, the transmitting station sends a Contention-Free-End (CF-End) frame. This is illustrated in FIG. 2B. If the TxOP is still left but the medium is no longer used, the transmitting station sends the CF-End frame to cancel the remaining TxOP. Thereafter, other stations which receive the CF-End frame contend again for medium access.
FIGS. 3A to 3C illustrate CF-End frames used for ensuring fair contention among stations for medium access.
As described above, in virtual carrier sensing, the CSMA/CA can be effectively used only when the MAC protocol data unit/PHY service data unit (MPDU/PSDU) are analyzed without errors. That is, virtual carrier sensing can be carried out only when a MAC header value can be correctly read. However, if data is sent using a high data transfer rate and an error occurs due to an unstable channel condition, or a receiving station cannot cope with the high data transfer speed, virtual carrier sensing cannot be carried out because the received MPDU/PSDU cannot be analyzed. Therefore, the CSMA/CA method becomes ineffective. Accordingly, when a legacy station which operates in accordance with the IEEE 802.11 a/b/g standard and a high throughput (HT) station having a higher capability than the legacy station coexist in the wireless LAN, and when a HT format is sent, the legacy station cannot analyze the HT format frame, causing ineffective operation of a CSMA/CA mechanism. The HT station may be a multi-input-multi-output (MIMO) station which has data transferring capability superior to a station operating in accordance with the 802.11 a/b/g standard.
In order to solve the above problems, an IEEE 802.11n standard has been in development. Referring to FIG. 3A, in the 802.11n standard, if HT stations and legacy stations coexist in the wireless LAN, a PHY header of the HT format frame is used as a legacy format (L-Preamble, L-SIG) so as to be recognized by the legacy stations. In addition, a RATE field value and a LENGTH field value are determined so that the legacy stations can recognize a time required after an L-SIG field begins and until an ACK frame is received, by analyzing the RATE field and the LENGTH field included in the PHY header. Hereinafter, the time specified by the RATE field and the LENGTH field will be referred to as an extended PHY protection (EPP).
When the EPP is used, medium access collision can be avoided, but stations have to contend unfairly to attain permission for medium access.
Referring to FIG. 3B, which illustrates the above problem, even if a legacy station can read the PHY header, the legacy station cannot read the next fields, that is, HT format, which leads to an error. Then, the PHY, or baseband layer, indicates the error occurrence to the MAC layer. The error indication begins where the EPP ends. From this point, the legacy station participates in contention for medium access after waiting for a longer time than the HT station. When an error occurs because the legacy station cannot read the HT format frame, the legacy station starts to back-off after standing by for a time defined in extended inter-frame space (EIFS, 94 us in IEEE 802.11a). This is different from the HT station which starts to back-off after standing by for a time corresponding to Distributed Coordination Function (DCF) inter-frame space (DIFS, 34 us in IEEE 802.11a).
FIG. 3C illustrates a CF-End frame used for solving this unfair contention problem. The receiving station broadcasts a CF-End frame when a short inter-frame space (SIFS) elapses after an ACK frame for a HT format data frame is received. Since the CF-End frame indicates that the medium is available, the HT station which receives the CF-End frame immediately participates in the contention for medium access without having to wait until the EIFS time elapses.
Accordingly, correct CSMA/CA can be carried out in the wireless LAN by using the CF-End frame, thereby ensuring a fair contention among stations. However, if the CF-End frame is not sent to all stations included in the wireless LAN, such advantages cannot be attained. As shown in FIG. 4, a hidden node problem may occur in the wireless LAN. In FIG. 4, a station A broadcasts a frame. Radio wave coverage of each station is indicated by a dotted line. Referring to FIG. 4, according to a feature of the wireless LAN, a station C can recognize a frame sent from the station A, but a station B may not recognize the frame. In this case, if the station B mistakenly recognizes that a medium is available by carrier sensing and thus tries to send the frame, medium access collision occurs.